First I created a Task using below link: https://mail.google.com/tasks/canvas
Then I marked it as Completed. When I checked the API Response for the same using: Services > Tasks API v1 > tasks.tasks.list [Returns all tasks in the specified task list.] I was able to view the updates & found the task i marked as complete.
However when I did the same using New interface (theme) from GMail, I found that the task I updated with completion was not at all there in above API Response.
Thus Google Tasks API does not give update about Task completion when New Gmail Theme used. Is there anything I missed or is it bug from Google Task API with Newly introduced theme?